This invention concerns a descaling device employing water.
The descaling device employing water according to the invention is used advantageously to remove the layer of oxides formed on the surface of the blooms or slabs immediately downstream of the mould, downstream of the induction furnace or immediately upstream of the rolling mill stands.
The invention is especially suitable for thin slabs or in all cases of the movement of slabs at a low speed, for instance when the continuous casting plant or the heating furnace is located in direct cooperation with the rolling line.
The device is employed advantageously with thin slabs between 20 and 80 mm. thick or with slabs or blooms being fed at a speed between 1.5 and 20 meters per minute, but advantageously between 4 and 10 meters per minute.
Various methods are known for the removal of the scale which forms on the surface of metallic workpieces during casting or heating upstream of the hot deformation, or during heat treatment, of those workpieces.
These descaling methods are divided substantially into mechanical methods, chemical methods and chemical-mechanical methods, depending on how they are carried out.
A method of removal of scale by high-pressure jets of water is generally used in rolling mills, the jets being directed at a suitable inclination against the slabs being fed forwards.
According to this method the faces to be descaled of moving blooms or slabs are continuously lapped by jets of water emitted by stationary nozzles at a pressure of about 12-40 MPa.
This method is unsuitable for the descaling of blooms or slabs being fed at a low speed since these jets of water under pressure cause excessive cooling of the bloom or slab.
This unfavourable effect is especially marked where the thin slabs have both their wide faces lapped by the continuous flow of descaling water.
Moreover, an efficient descaling action requires a given relative speed between the delivery jet and the bloom being fed and a considerable rate of flow of water divided between a great number of nozzles; for instance about 800 liters per minute split between 36 nozzles, are required to achieve efficient descaling of blooms having a square cross-section with sides of 280 mm.
The high rate of flow of water, besides the waste involved, entails the problem of generating a great quantity of steam when the water strikes the bloom.
The great quantity of water required involves also the employment of very powerful feed pumps and pipes of great sizes.
Moreover, owing to the excessive cooling the bloom has to be re-heated before undergoing the rolling and subsequent processes.
This method is therefore effective only in rolling plants of a discontinuous type where the rolling speed is high enough, but is unacceptable in continuous rolling plants where the rolling speed is the same as or close to the casting speed and is therefore especially slow.
EP-A-0484882, U.S. Pat. No. 3,511,250, FR-A-2.271.884 and the Patent Abstract of Japan, Vol. 8, No. 230 disclose descaling devices employing water in which the nozzles are moved, for instance by rack and pinion systems or piston-cylinder systems, in a direction crosswise to the direction of feed of the slab; but these devices do not include periods of stoppage of the delivery of water and therefore entail wastage, generation of great quantities of steam, excessive reduction of the temperature of the slab, etc.
In particular, EP-A-0484882 arranges to descale the two opposed faces of a slab between 200 and 240 mm. thick with counterpart Jets of water. This system creates concentrated surface cooling, which becomes especially unfavourable near the edges, which already tend to be too cool.
GB-A-1,071,837 and DE-B-2.605.001 include nozzles able to move to and fro on the axis of the slab being fed and provide for stoppage of the water during the return movement of the nozzles to their position of re-starting the cycle, but do not explain the reason for this stoppage.
GB-A-1,071,837 in particular concerns cylindrical bars and especially hollow cylindrical bars, includes a plurality of delivery nozzles and does not mention the values of pressure or rate of flow of the water which characterise the device.
DE-B-2.605.001, which is dated eleven years later, provides for the descaling of slabs with a jet of water, which is moved alternately to and fro in the direction of feed of the slab, substantially as in GB-A-1,071,837.